1. Field of the Invention
The present invention relates to a small size discharge lamp, and more particularly, to a small size fluorescent lamp of a bar type having, for example, an outer diameter smaller than 5 millimeters, a length less than 300 millimeters and a power less than 10 watts.
2. Description of the Prior Art
Recently, a television receiver which uses a liquid crystal in the screen unit (liquid crystal television) has been developed, and some models of such a receiver have been already released in the market as a pocket size television or a wall handing type television. The screen for such a television receiver is defined by liquid crystal panel. To provide a sufficient luminance of the screen, one or more small size fluorescent lamps, known as the back light, are provided behind the liquid crystal panel. The liquid crystal color television receivers now being released are of two to three inch type, but recently sizes up to about 12 inches have been developed. These television receivers are mostly battery operated and, therefore, it is preferable to operate them with less power. Also, since liquid crystal itself does not produce any light, it is necessary to provide a light source, which must be sufficiently small to fit behind the liquid crystal panel. Furthermore, the operation of the light source should be stable under various conditions and produce a constant light.
According to the prior art small size fluorescent lamp, the outer diameter of the tube is usually greater than 7 miilimeters, having a relatively large heat capacity. Thus, when the heat generation effected at the electrodes is low, i.e., when the power supplied to the lamp is low, the tube will be heated very moderately, resulting in an unstable operation of the lamp, particularly when the ambient temperature is less than, e.g., 5.degree. C. If the temperature falls below 5.degree. C., the temperature of the tube itself does not rise much more than 5.degree. C. Thus, the pressure of the mercury vapor inside the tube will fall thereby deteriorating the luminous efficiency. This will result in an insufficient brightness for the back light.
In many fluorescent lamps, a getter, defined by a plate deposited with mercury, is placed behind the electrodes, i.e., at a space between the electrode and the end of the tube, for enclosing the mercury vapor and also for absorbing unwanted impurity gas generated during the discharge. When this arrangement is employed in a fluorescent tube having a length longer than 400 millimeters, the percentage of the distance between the opposite electrodes with respect to the entire length of the tube is still high, thereby providing a sufficient length of arc between the electrodes. However, when the same arrangement is employed in a small size fluorescent tube having a length shorter than 300 millimeters, said percentage becomes relatively low, resulting in an insufficient length of arc between the electrodes, compared to the total length of the tube.
Furthermore, if the electrode is made of a filament coil, its length should be longer than 3 millimeters. Since 1 millimeter is necessary for the electric connection with a lead wire at each end of the filament coil, the electrode extends with no extra space when it is arranged to be perpendicular to the axial direction of the tube, which has an inner diameter of 5 millimeters. In other words, when the inner diameter of the tube is less than 5 millimeters, the filament coil can not be arranged in the above described manner.
More over, according to the prior art fluorescent lamps, each of the opposite end caps for socketing tube has two terminals. It is preferable to reduce the number of terminals to one to simplify the structure of the end cap.